Multiple spool gas turbine engines are generally known in the art, wherein at least two turbocompressor rotating groups are provided in association with a combustor. Each turbocompressor rotating group comprises a compressor stage and a turbine stage mounted on a common spool or shaft, with the shafts of separate rotating groups being arranged in concentric relation to each other. In a typical twin spool engine, a high pressure spool includes a compressor stage and a turbine stage disposed on opposite sides of the engine combustor, and rotatably interconnected by a hollow shaft which rotatably receives the shaft of a low pressure spool including a compressor stage and a turbine stage. In operation, the compressor stages of the low and high pressure spools provide dual stage compression of air which is supplied to the combustor for combustion with a suitable fuel. The hot gases of combustion are then expanded in series through the turbine stages of the high and low pressure spools, respectively, to provide an engine power output. One advantage of multiple spool gas turbine engines of this general type is that such engines can accelerate rapidly in order to accommodate increased power output requirements.
Gas turbine engines of the multiple spool type include a significant number of rotating and related bearing components which require lubrication for continued engine operation. In this regard, oil lubrication systems are well-known for delivering lubricant to selected bearings and related structures throughout the engine. Sump seals having a labyrinth or similar configuration are normally provided to prevent leakage of lubricating oil into the main flow path of air and combustion gases through the engine. Buffer seal arrangements have been proposed to pressurize engine sump seals in order to decrease the likelihood of oil leakage.
It is a common practice in gas turbine engines to supply a small quantity of air from the flow path to buffer oil sump seals at various locations throughout the engine. FIG. 1 shows a common arrangement for buffering an oil sump seal. In this arrangement, pressurized air 1 from the engine is delivered to an annulus 2 in between the buffer labyrinth seal 9 having an aft portion 3 and a forward portion 4. The aft portion has three knife seals while the forward portion 4 has only a single knife seal and a slinger 5. A first portion of the air from annulus 2 flows forward through the forward portion 4, through an oil sump carbon ring seal 6 and to the oil sump 7, while a second portion of the air flows across the three knife seals into a cavity 8. The purpose of providing a buffer seal 9 adjacent the oil sump seal 6 is to provide an adequate air-to-oil differential pressure across the oil sump seal 6 at all points in the flight envelop so that oil leakage across the sump seal 6 is prevented. In spite of all the care exercised in designing oil sump seals and providing adequate buffer air pressure, oil sump seals have been known to still leak oil at some point during the life of the engine. This oil leak can be due to an excessively worn out sump seal, a cracked sump seal, a coked sump seal, low or reverse differential pressure during transient or some steady state point in the operating envelop. If this oil leak location is forward of the bleed air port location in the engine gas flow path, this oil leak can contaminate the bleed air. The oil contamination of bleed air can cause an unpleasant odor in the cabin as this bleed air is used to pressurize the aircraft cabin. This is an unacceptable scenario which may result in an inflight shutdown or unscheduled removal of the engine from the airframe.
Accordingly, there exists a need for further improvements in bleed air buffer seal arrangements for use in all types of gas turbine engines to positively prevent sump seal oil leakage from contaminating the bleed air throughout the range of normal engine operating conditions.